Military aircraft that penetrate hostile air space become targets for radar directed weapons, such as antiaircraft cannon fire, guided missiles, and airborne fighter interceptors. Systems for countering hostile radar are necessary in order to avoid intercept. One approach to providing such a counter system is to mimic the hostile radar's transmission to generate a false target or targets to confuse the hostile radar. The broad area of mimicking the hostile transmission is known as "spoofing".
One type of spoofing is range gate pull-off. This method involves reception of radar signals by repeater deception jammers and retransmission of the signals delayed in time. Increasing the time delay steadily, from retransmission to retransmission, causes the target echo to appear to move away from the target itself. This is effective when the tracking radar is of a type which places a tracking range gate that centers on an echo from the target and automatically follows it as the target moves in range. If a repeater generates a stronger echo than that of the target itself, and delays the generated echo in time, the tracking range gate will tend to lock onto the repeater signal instead of the echo from the target.
Another form of spoofing that works in a manner similar to range gate pull-off is known as Doppler (velocity) gate pull-off. This type of spoofing is effective when the hostile radar is of a type which tracks targets in Doppler filter banks. The radar signal is received by the repeater and translated in frequency and retransmitted. As in the case of range gate pull-off, if the retransmitted signal is stronger than the echo of the target itself, the hostile radar will tend to lock onto the repeater signal, and the target echo will appear to move away from the actual target.
Most repeater methods for confusing hostile radar depend on a receiver and signal processing system on the aircraft to intercept signals from the hostile radars and take appropriate action, and on an on-board antenna for transmitting a false echo. The receiver/signal processor sorts out the frequency of the intercepted signal and determines the radar type and intended mission. The hostile signal can be stored, delayed and/or Doppler shifted, and retransmitted to appear to the radar as an echo from a real target, delayed in range from the real target, i.e. at a greater range than the real target. Sophisticated systems make use of high fidelity memories to store and retransmit an exact replica of the received signal. This type of high fidelity capability makes it possible to mimic the radar exactly even when the radar impresses sophisticated modulation on the signal.
A repeater jammer which spoofs radar by simply delaying the retransmission in time to create range deception or changing the frequency of retransmission to create velocity deception has a significant limitation in that the signal is emanated from the aircraft being defended. Therefore, there is no spoofing of spatial angular information regarding the target's whereabouts. A known technique for overcoming this limitation is to tow the repeater transmitter behind the aircraft. This technique is effective for spoofing spatial angular information but, in current conventional technology, has presented a number of problems. If the repeater is an active transmitter, it must be provided with power through a link between the aircraft and the repeater. The receiver and signal processing equipment are relatively expensive, complex, and heavy and, thus, are usually located on board the aircraft. Therefore, radio frequency (RF) signals are sent over a transmission line from the aircraft to the repeater. The best transmission line for transmitting RF signals is normally coaxial cable. Such cable has a number of drawbacks. It is heavy and bulky and has a high loss rate at the frequencies commonly used by intercept radars. These drawbacks preclude the use of long lengths of coaxial cable (e.g., longer than 100 meters) for towing a repeater transmitter. The limitation on the length of the link between the repeater and the aircraft limits the angular deception that can be achieved with conventional towed repeaters.
There are additional disadvantages to the use of metallic wire cables for transmitting RF signals and power to a towed repeater. The cables themselves can have a significant radar cross section at certain aspect angles. This characteristic is particularly disadvantageous when the towing aircraft is a stealthy aircraft with a low radar cross section. Longer metallic wire cables can also present a navigation hazard to other aircraft flying in the vicinity of the protected aircraft. In addition, an electromagnetic pulse could induce a significant amount of energy in such a cable and damage components on board the aircraft or the decoy.
Fiber optic technology makes possible long distance transmission of radio frequency (RF) energy over lightweight optical fibers. The transmission of the energy can be accomplished very efficiently since optical fibers are capable of high bandwidth bidirectional transmission of RF signals with very low loss. Fiber optic technology has been used in various fields. One known use is in connection with fiber optic guided missiles. The depolyment of such missiles has demonstrated that it is possible to pay out long lengths of optical fiber from the missiles. Lengths of optical fiber up to twenty kilometers have transmitted signals from a guided missile to a ground station. The use of fiber optic technology in connection with guided missiles has also helped spur development of high strength optical fibers with pull strengths exceeding 400 kpsi (thousand pounds per square inch). This is more than the tensile strength of a steel wire. An ordinary commercial grade glass fiber has about 100 kpsi proof strength.
U.S. Pat. No. 4,808,999, granted Feb. 28, 1989, to D. Toman, discloses a towed decoy adapted to be towed behind an aircraft. Signals are transmitted to the decoy from the aircraft by a fiber-optic cable. The cable is described as preferably being incorporated at the time of manufacture directly into a larger cable used for towing the decoy. Receiving apparatus on the aircraft generates a desired excitation signal, which may be a distorted replica of the received radar signal, and modulates the signal upon a laser transmitter. The modulated laser signal is transmitted to the decoy via the fiber-optic link. A laser receiver on board the decoy demodulates the laser signal and transmits a derived RF signal to a transmitter on the decoy which amplifies the signal and transmits it via an antenna. Direct current energy is supplied to the decoy electrical components by a battery on board the decoy. Toman states that the techniques generator on board the aircraft may produce any number of desired deception techniques, including velocity pull-off and range pull-off.